Boride cathodes



. 1500 degrees C. I have Patented Nov. 17, 41953 A:4559.685 norman camiones James M. Lamm, Schenectady. N. Y., 'assigner to General Electric Company. a corporation of A New York Application august s, reso. sum1 ivcrzazoi somma. (cana-31.5)

My invention relates to improved boride cathodes and is in the nature of an improvement on the invention described and claimed in my copending application Serial No. 153,212, nled March 31, 1950, now matured into Patent No. 2,639,399 granted May 19, 1953 and assigned to the assignee of this application.

In the above application it is disclosed that the rare earth metal borides including misch metal boride. the alkaline earth borides, and thorium and uranium borides provide electron emitters having very desirable electrical and chemical properties. These materials are in general capable of operating at high temperatures and I have found that inthe operation of cathcdes consisting essentially of a coating of one of these borides or a mixture of these borides on a refractory metal support or core that the life of the vcathode is relatively short. I have also found that the life of these cathodes is materially -lengthened by providing a suitable material between the core metal and the boride coating. It appears that the short lifeexperienced with cathodes made by applying the borides directly to a refractory metal core results from the reduc- -tion of' the borides and the diffusion, at the relatively high-temperatures, of the boron into the refractory metal, leaving the metal of the boride free to evaporate from the cathode surface at a rapid rate.

In accordance with an important aspect of my invention a suitable refractory core metal such l.UNITED STAT-Es zPa'raN'r OFFICE,A

. stable at high temperature and cathodes having sired. In a particular as tungsten, molybdenum, platinum, columbium or tantalum is treated to provide at least a surface coating of a material into which the boron of the boride coating will not diffuse substantially at the operating temperature of the boride cathodes which may be, for example, in the order of found that treating the 'core metal by boronizing, carburizing or nitrding is effective for this' purpose and of these treatments the carburizing is preferred. As will be readily appreciated by those of boronizing the surface that the operation of the cathode at high temperaturewill cause further diifusion of the boron which is followed by a diffusion of the boron of the boride coating into the surface of the core metal. For this reason when boronizing isl employed it is necessary to completely boronize the core material with the result that it is rather brittle, For this 'reason the nitriding and carburizing are preferred and of these the carburizing is superior. The carbides in general are very skilled in, the artif the boronizing is carried out only to the extent into the core metal a surface of carbide may be operated at elevated temperatures, in the order of 1500 degrees C. without any substantial reaction between the boride coating and the carbide. Of the refractory Y metal carbides, tantalum carbide is the most stable and provides the most desirable overlayer for the cathode core metal.

In its broader aspects my invention contemplates an interposed layer of graphite between the core metal and the boride emitting material or a graphite support for those cathodes in which a. metal support is not required.

My invention will be better understood by oonsideration of a specic cathode embodying my invention,` reference being had to the accompanying drawing in which ig. 1 is an elevational view of a cathode prepared in accordance with my invention; Fig. 2 is an enlarged elevational view in section of a portion of the cathode of Fig. 1; and

Fig. 3 is a schematic representation of a modified cathode construction embodying my invention.

Referring now to the drawing, the cathode proper includes a core wire I which may be of tantalum although the other refractory metals previously mentioned may be employed if decathode constructed in accordance with the invention the core wire was of .020 inch in diameter and it was provided with overwndlng 2 of tantalum wire of .010 inch in diameter vwith the turns of the over-winding spaced apartby .010 inc The space between the overwound turns was filled with a rare earth bende awww app ying the bori e coa ing t e tantalum core and overwinding was carburized by packing ln finely powdered sugar charcoal in a graphite crucible and heated in vacuum for one minute at'2300 degrees C. This provided a surface layer of tantalum carbide having a thickness in the order of 2 mills and illustrated at l' and 2" in Fig. 2. The center of a tantalum core remained uncarburized and relatively ductile so that the l resultant structure is not too brittle to be commercially practicable. The boride coating was then applied over the tantalum carbide coating on the core and overwinding. The cathode thus constructed was operated at a temperature .of I

,boride the alkaline earth boridea, and thorium Athe boronizing may be carried out in substantially the same manner as the carburizing except that a powdered boron powder is employed instead of the finely divided charcoal. Also the nitriding process is well known in the art and may be carried out by subjecting the core material to nitrogen or ammonia at an elevated temperature in the order of 200D to 2500 degrees depending upon the core metal employed.

While the carbide surface described above is u preferred it is to be understood that other forms of carbon such as graphite may be employed as.

an intermediate layer between the core metal and the boride coating.

In Fig. 3 I have shown a modification ofmy invention in which the cathode is indirectly heated and includes a graphite support without any core metal. Referring to Fig. 3, there is shown schematically the electrodes of a planar type electric discharge device including an anode 1, a screen grid 8, a control grid 9 and the cathode III, which is in the form of a graphite disk having a rim II. The rim and disk define a recess which is filled with a boride emitter I2.

l The cathode is heated by radiation from a heater c'oil I3 and may also be heated by electron bombardment if the coil I3 is maintained at a suitable negative potential with respect to the cathode III.

By making the support for the boride emitter of graphite the life of the cathode is greatly improved. The tendency of the boron to diffuse into the support is essentially eliminated in the same manner as it is in connection with the modification of my invention where the core wire' is carbonized. It is apparent that an interposed layer of graphite may be employed in a cathode including a core wire and a boride emitter.

While I have described particular embodiments of my invention it will be apparent to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects and I aim in the appended claims to cover all such changes and modifier sentialiy of carbon.

tions as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

i. A cathode comprising a refractory metal core. an electron emissive coating thereon consisting essentially of a rare earth metal boride and an interposed layer including the carbide 01 the core metal for minimizing the diffusion of boron from the coating into the core metal.

2. A cathode comprising a tantalum core, an

electron emissive coating thereon consisting essentially of a maib/oride selected from the group consisting of the rare earth metal borides and mixtures thereof. an interposed layer of tanta- 1um`clr1/bige'for minimizing the diiusion of boron.

from e coating into the tantalum core.

3. A cathode comprising a refractory metal core. an electron emissive coating thereon comprising a metal boride and an interposed layer consisting essentially of carbon .for minimizing the diiiusion of boron from the coating into the core metal.

4. A cathode comprising a filtalum core, an electron emissive coating thereon comprising l. metgahboride and an interposed layer consisting essentially'of carbon for minimizing the diffusion of boron frnthe coating Into the tantalum core.

5. A cathode comprising a refractory metal core, the metal being selected from the group -consisting of tungsten, molybdenum, platinum.

columbium and tantalum, an electron emissive coating thereon comprising a metal boride, selected from the group consisting of the rare earth metal borides and mixtures thereof. and an interposed layer selected from the group consisting of the carbide, niwtrjicie, and borde of the core metal.

6. A cathode comprising a body of emitting material consisting essentially of a metal boride and a refractory metal support for said emitting material having at least the surface thereof contiguous with the emitting material consisting es- JAMES M. LAFFERTY.

Reference; cned minne ale'of this patent UNrn-:n sTATEs PATENTS Number Name Date 2,438,732 Williams Mar. 30, 1948 2,447,973 Williams Aug. 24, 1948 

1. A CATHODE COMPRISING A REFRACTORY METAL CORE, AN ELECTRON EMISSIVE COATING THEREON CONSISTING ESSENTIALLY OF A RARE EARTH METAL BORIDE AND AN INTERPOSED LAYER INCLUDING THE CARBIDE OF THE CORE METAL FOR MINIMIZING THE DIFFUSION OF BORON FROM THE COATING INTO THE CORE METAL. 